In the course of the successive worldwide promotion of regenerative energies, and specifically the intensive use of wind energy, increased demands are being made on the part of the power supply companies (EVU) on the communication of wind farms. Wind farms can be composed of several 100 wind turbines (=slaves), wherein a farm master monitors and controls the wind farm. The farm master thereby has the function of supplying the respective slaves (for example, programmable control) of the individual wind turbines with data at specific time intervals (cycles).
Standard protocols such as real-time Ethernet as well as other Ethernet-based field bus protocols cannot satisfy the increased requirements ((for example, operate 500 wind turbines in a millisecond cycle (typically 5-20 ms depending on the performance of the wind farm management system) and thereby exchange over 100 useful data bytes bi-directionally)).
A structure of a wind farm group grid system is usual in which a computing unit (farm master) is connected to a farm network via signal paths and switches. Parts of a wind farm can thereby comprise sub-networks with respectively up to 50 units of wind turbines, which in turn communicate with one another via a superordinate network. Each wind farm group feeds the data generated by the individual wind turbines from the sub-network via a data coupling module (switch) into the farm network. A plurality of sub-networks can thus connect to the farm network, wherein each sub-network is composed of up to 50 individual wind turbines. However, wind farm group grid systems are also conceivable which form a network without a sub-network with up to 500 wind turbines.
Tests have shown that there have not been any data transmission protocols so far that permit a high-speed communication with the quantity structures and topologies described above. The classic Ethernet with its TCP/IP protocol is not designed for these large data quantities and the required real-time behavior.
It must be taken into account that each wind turbine receives and transmits a data flow of, for example, 100 to 300 bytes in each cycle. An optimal cycle time would hereby be 20 milliseconds or less. The cycle time may be exceeded or fallen below only negligibly (real-time behavior).
The known real-time Ethernet protocols are not suitable either for processing these large data quantities and number of subscribers in the specified cycle time. Moreover, these protocols do not have any way of being operated with security mechanisms such as encryption and authentication, which also eliminates the known real-time Ethernet protocols.
Tests have shown that the standard network protocol is not sufficiently deterministic because the multiple-layer model of the TCP/IP protocol (OSI) composed of 7 layers causes a marked delay of the data transmission, which in the present case with the high demands made, is no longer sufficient for the data transmission. The TCP/IP protocol also has the problem that the data packets cannot be prioritized. A preferred treatment of certain real-time data is therefore not possible.
A layer model of the International Organization for Standardization (OSI) is referred to as an OSI layer model (also OSI reference model: Open Systems Interconnection Reference Model). It was developed as the design basis for communications protocols.
The functions of the communication to this end were divided into seven layers built up one on top of the other. For each layer there is a description that states what this layer has to accomplish. These requirements have to be realized by the communication protocols. The concrete implementation is not stipulated thereby and can therefore be very different. Thus in the meantime numerous protocols of this type exist for each layer.
In the case of the OSI model, there are seven layers with fixed requirements. On each individual layer in each case an instance implements the requirements. The instances on the transmitter and receiver side must operate according to fixed rules so that they agree how the data are to be processed. The establishment of these rules is described in a protocol and forms a logical horizontal connection between two instances of the same layer.
Each instance provides services that can be used by a layer directly above it. To provide the service, an instance itself uses the services of an instance directly below it. The real data flow thus occurs vertically. The instances of a layer can be exchanged precisely when they are exchanged at the transmitter as well as with at the receiver.
The Seven Planes of the Prior Art OSI Layer Model
The degree of abstraction of the functionality decreases from layer 7 through layer 1.
Layer 7—Application Layer
The application layer (also: processing layer) is the topmost of the seven hierarchical layers. It provides the applications with access to the network (for example, for data transmission, e-mail, virtual terminal, remote login, among others). The actual application process lies above the layer and is not handled by the OSI model.
Hardware/software on this layer: Gateway, protocol converter, fax to e-mail services.
Protocols and standards: X.400, X.500, ISO 8571 (FTAM), ISO 9040/9041 (VT), ISO 9506 (MMS), MHS, VTP, FTP, NFS, Telnet, SMTP, HTTP, LDAP, JTM, SSH.
Layer 6—Presentation Layer
The presentation layer (also data presentation layer, data provision plane) converts the system-dependent presentation of the data (for example, ASCII, EBCDIC) into an independent form and thus renders possible the syntactically correct exchange of data between different systems. Also functions such as data compression and encryption belong to layer 6. The presentation layer ensures that data that are transmitted by the application layer of a system can be read by an application layer of a different system. If necessary, the presentation layer acts as a translator between different data formats, in that it uses a data format, the ASN.1 (Abstract Syntax Notation One), which can be understood by both systems.
Protocols and standards: ISO 8822/X.216 (Presentation Service), ISO 8823/X.226 (Connection-Oriented Presentation Protocol), ISO 9576 (Connectionless Presentation Protocol).
Layer 5—Session Layer
Layer 5 (Session Layer, control of logical connections) guarantees the process communication between two systems. Here, among other things, the RPC (Remote Procedure Call) protocol is found, in order to eliminate session breakdowns and similar problems, the session layer provides services for an organized and synchronized exchange of data. For this purpose reset points, so-called check points, are introduced, at which the session can be synchronized again after the loss of a transport connection without the transmission having to start over again.
Protocols and standards: ISO 8306/X.215 (Session Service), ISO 8327/X.225 (Connection-Oriented Session Protocol), ISO 9548 (Connectionless Session Protocol), RPC (Remote Procedure Call).
Layer 4—Transport Layer
The functions of the transport layer (also: end-to-end control, transport control) include the segmentation of data packets and congestion avoidance. The transport layer provides the application-oriented layers 5-7 with a uniform access so that they do not need to take into account the prosperities of the communications network. Five different service classes of different quality are defined in layer 4 and can be used by the upper layers from the simplest to the most sophisticated service with multiplex mechanisms, error control and error correction methods.
Protocols and standards: ISO 8073/X.224, ISO 8602, TCP, UDP, SCTP.
Layer 3—Network Layer
The network layer (also: packet layer) guarantees with line-oriented services the switching of connections and with packet-oriented services the forwarding of data packets. The data transmission goes in both cases respectively beyond the entire communications network and includes the routing between the network nodes. Since direct communication between the sender and the target is not always possible, packets must be forwarded by nodes that lie on the way. Forwarded packets do not reach the higher layers, but are provided with a new intermediate target and sent to the next node.
The most important functions of the network layer include the establishment and updating of routing tables and the fragmentation of data packets. In addition to the Internet Protocol, the NSAP addresses also belong to this layer. Since a communications network can be composed of several partial networks of different transmission media and protocols, in this layer the implementation functions are also found, which are necessary for forwarding between the partial networks.
Hardware on this layer: router, layer 3 switch (Brouter).
Protocols and standards: X.25, ISO 8208, ISO 8473 (CLNP), ISO 9542 (ESIS), IP, IPsec, ICMP.
Layer 2—Data Link Layer
The object of the data link layer (also: section security layer, data security layer, connection security layer, connection plane, procedure plane) is to ensure a reliable, that is, a largely error-free transmission and to regulate access to transmission medium. The division of the bit data flow into blocks and the addition of sequential numbers and checksums are used for this purpose. Faulty, incorrect, or lost blocks can be requested anew by the receiver by means of acknowledgment and repetition mechanisms. The blocks are also referred to as frames.
A “data flow control” makes it possible for a receiver to dynamically control the speed at which the opposite side may send blocks. The Institute of Electrical and Electronics Engineers, or IEEE, saw the necessity to also regulate collision resolution for a transmission medium for local networks, which is not provided in the OSI model.
According to the IEEE, layer 2 is divided into two sublayers: LLC (Logical Link Control) and MAC (Media Access Control).
Hardware on this layer: bridge, switch (multiport bridge).
The Ethernet protocol describes layer 1 as well as layer 2, wherein on this CSMA/CD is used as access control.
Protocols and standards that are based on other layer 2 protocols and standards: HDLC, SDLC, DDCMP, IEEE 802.2 (LLC), ARP, RARP, STP.
Protocols and standards that are based directly on layer 1: IEEE 802.11 (WLAN), IEEE 802.4 (Token Bus), IEEE 802.5 (Token Ring), FDDI.
Layer 1—Physical Layer
The physical layer is the lowest layer. This layer provides mechanical, electrical, and further functional auxiliaries in order to activate or deactivate physical connections, to maintain them, and to transmit bits via them. This can be, for example, electric signals, optical signals (fiber optics, laser), electromagnetic waves (wireless networks), or sound. The methods used for them are referred to as transmission processes. Devices and network components that are assigned to the physical layer are, for example, the antenna and the amplifier, plug and socket for the network cable, the repeater, the hub, the transceiver, the T connector, and the terminator.
On the physical layer the digital bit transmission is carried out on a line-based or lineless transmission segment. The common use of a transmission medium can take place on this layer by means of static multiplexing or dynamic multiplexing. In addition to the specifications of certain transmission media (for example, copper cable, optical waveguide, electric mains) and the definition of plug connections, this also requires further elements. Moreover, on this plane it must be decided how an individual bit is to be transmitted at all.
This is defined as follows: in currently known computer networks information is usually transmitted in the form of bit sequences. Of course, the values 0 and 1 are unknown to the physical type of transmission itself, for example, voltage pulses in a copper cable in the case of electric transmission, or frequencies and amplitudes of electromagnetic waves in the case of radio transmission. Therefore for each medium a coding of these values must be found, for example, a voltage pulse of a specific level or a radio wave with a specific frequency, in each case based on a specific duration. These aspects must be precisely defined for a specific network. This takes place with the aid of the specification of the physical layer of a network.
Hardware on this layer: modem, hub, repeater.
Protocol and standards: V.24, V.28, X.21, RS 232, RS 422, RS 423, RS 499.
The network protocols developed according to the OSI reference model have in common with the TCP/IP protocol family that these are hierarchical models. However, there are minor conceptional differences. OSI establishes the services precisely, which each layer has to provide for the layer above it. TCP/IP does not have the same kind of strict layer concept as OSI. The functions of the layers are not precisely established, nor are the services. It is permitted for a lower layer, going around interim layers, to be used directly by a higher layer. TCP/IP is thus considerably more efficient than the OSI protocols. The disadvantage of TCP/IP is that there is an individual network protocol in each case for many small and micro services. In contrast, OSI has established a large scope of services, which has many options, in each case, for its protocols. Not every OSI software commercially available has the full scope of services implemented. Therefore OSI profiles were defined, each of which contain only one specific set of options. OSI software from different manufacturers works together when the same profiles are implemented.
However, the OSI layer model is not the only reason why TCP/IP is not suitable for a real-time transmission. TCP/IP uses only the lower 4 layers. It is rather the fact that all network stacks, regardless of whether they are TCP/IP or real-time Ethernet protocols, require a large CPU capacity for the communication required in this application or do not have sufficient real-time behavior. Another reason for the lack of real-time capability of TCP/IP is the lack of prioritization of the data packets (VLAN tag) and the associated mixing of slow and high-speed communication.